1. Field of the Invention
This invention relates to a drum type image scanning and recording apparatus and, more particularly, to an apparatus for protecting a photosensitive material such as a photosensitive film (or an original picture) wrapped on a recording drum (or a scanning drum) and attached to the drum by vacuum to prevent separation of the material from the surface of the drum as the drum rotates.
2. Description of the Prior Art
In an image scanning and recording apparatus of a cylindrically scanning type, particularly in such apparatus used for graphic arts or the like, the following steps are taken for attaching a photosensitive material such as a film, a photographic paper or the like (referred to as "film" hereinafter) to a drum in a recording unit, or for attaching an original picture to a drum in a scanning unit: perforating several holes in a line near the leading edge of the film; fitting said holes to pins (preferably the same number as the holes) aligned on the external surface of the drum and in parallel with the drum axis; tensionally wrapping the film on the drum by pulling a free or unpinned edge of the film; attaching securely the ending edge of the film to the drum using an adhesive tape or a press bar; and, at the same time, vacuum suctioning the film by way of small holes penetrating from the surface to the interior of the drum, thus positively attaching the film to the external surface of the drum.
However, in such a method perforating of the film is necessary, and so is the process of securely attaching the ending edge of film to the drum by means of an adhesive tape or a press bar if the process of tensionally attaching the film to the recording drum is to be automated. These processes pose serious technical obstacles in automatization. In addition, the perforated part and the taped part of the film cause a reduction in the effective area of the film; when the pins are fitted to the holes, the film becomes deformed; and, when the film is taped, the adhesive tends to stick to the surface of the recording drum, thereby contaminating film used later on the same drum. To eliminate such inconveniences, and to make the process of tensionally attaching the film to the recording drum amenable to automatization, it is desirable that the pins on the external surface of the recording drum and the method for attaching the film by taping or the like both be omitted.
Instead, the film can be tensionally attached securely to the recording drum solely by vacuum suctioning through small holes perforated on the external surface of the recording drum.
However, when the film is tensionally attached to the drum solely by vacuum suction force and this force is reduced or lost suddenly while the drum is rotating the film will immediately separate away from the external surface of the drum by centrifugal force because there are no other retaining means e.g., pins or tape. After such an event, even if the film is rewrapped on the drum, it is difficult to return the film to its original wrapped position and may become useless. Moreover, such an event can lead to damage to expensive components, e.g., the drum surface or a focussing lens of a recording head.
In view of the above problem, the applicant has proposed a drum-type image scanning and recording apparatus, as disclosed in Japanese Patent Application No. SHO 58-155664, in which: a film is kept from separating away from a rotating recording drum even in an emergency such as a power failure, vacuum apparatus failure, or the like by providing that in such an emergency, the film is kept suctioned (by vacuum) to the drum external surface for a long time.
Even without an emergency or vacuum failure, separation of the film from the drum may occur because a rolled film itself has a propensity to curl; and complete or partial clogging of the holes perforated in the recording drum external surface may cause the film to float away from the drum surface, thus producing gaps therebetween, even during the initial stage of mounting the film on the recording drum. Among such causes that may make the film float, the propensity in the film itself to curl up is significant in rolled film that has been cut off at the part immediately adjacent the reel. This propensity of the rolled film is due to a stepwise part of the film which appears at the beginning part of the film when it is rolled on the reel. It is also due to the unevenness of the surface of the reel itself, or due to the smaller inside diameter of the reel. Such problems cannot be entirely avoided when a rolled film is used. Also, even when taping is used for attaching the film to the drum, the tape may come off the drum while it is rotating.
Accordingly, in order to prevent the film from separating away from the drum the latter has to stop rotating immediately when the film begins to lift away from the drum surface. The applicant has investigated this problem to detect the floating or lifting of the film, using several microswitches placed in the vicinity of the recording drum surface. However, floating could not thus be securely detected because the floating part of the film was easily flattened when pressed by the switches. Even when floating could be detected, the film got hooked by the detecting levers of the microswitches, thereby promoting the film separation. Subsequently, the applicant investigated an optical detecting means for detecting floating of the film in a noncontacting way in which a light beam is projected nearby and along the recording drum external surface, in parallel with the axis of the drum, said light beam being interrupted when the floating of the film occurs. However, in order to attain adequate detection accuracy such optical detection means requires complex and expensive apparatus, including the use of a laser oscillator as a light source. A need, therefore, exists for a simple and inexpensive solution that will hold film securely to a rotating drum.